USCIS will only accept the new Form N-400 Application for Naturalization as of 5/5/2014.
Why is there a new Form N-400?
The new form is 21 pages long and uses 2D technology. This means that USCIS prefers that you type your answers into the form and then print it out so it can easily scan the data into its database.
What if after 5/5/2014 I accidentally mail in the old form?
USCIS will reject your Form N-400 and will return your filing fees as well. You will have to re-file it on the new form and send in the filing fees again. Your application is not considered pending if the entire packet, including the filing fee, is rejected and returned to you.